1. Field of the Invention
The present invention relates generally to laser technology for photolithography, and more particularly to EUV dose control during laser firing.
2. Description of the Prior Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site.
The line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may be mixed or dispersed with another material such as a liquid. Delivering this target material and the laser beam simultaneously to a desired irradiation site (e.g., a primary focal spot) within an LPP EUV source plasma chamber for plasma initiation presents certain timing and control challenges. Specifically, it is necessary for the laser beam to be focused on a position through which the target material will pass and timed so as to intersect the target material when it passes through that position in order to hit the target properly to obtain a good plasma, and thus, good EUV light.
A droplet generator holds the target material and extrudes the target material as droplets which travel along an x-axis of the primary focal spot to intersect the laser beam traveling along a z-axis of the primary focal spot. Ideally, the droplets are targeted to pass through the primary focal spot. When the laser beam hits the droplets at the primary focal spot, EUV light output is theoretically maximized. In reality, however, achieving maximal EUV output light across bursts over time is very difficult because energy generated by irradiation of one droplet varies randomly from energy generated by irradiation of another droplet.
Thus, maximal EUV light output might sometimes—but not always—be realized. This variability in output is a problem for downstream utilization of the EUV light. For example, if variable EUV light is used downstream in a lithography scanner, wafers can be non-uniformly processed, with resultant diminution of quality control of dies cut from the wafers. Thus, a tradeoff of non-maximal EUV for greater reliability may be desirable.
A stroboscopic pattern produces EUV in short exposures throughout exposure of a wafer die. Although this pattern of bursts can be beneficial for control of the EUV energy dose, what is needed is a method to generate—with greater reliability—acceptable levels of EUV energy output for downstream purposes—that is, to more accurately control an EUV energy dose.